Immersed membranes are used for separating a permeate lean in solids from tank water rich in solids. Feed water flowing into a tank containing immersed membranes has an initial concentration of solids. Filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side of the membranes. As filtered water is permeated through the membranes and removed from the system, the solids are rejected and accumulate in the tank. These solids must be removed from the tank in order to prevent rapid fouling of the membranes which occurs when the membranes are operated in water containing a high concentration of solids.
In a continuous fully mixed process, there is typically a continuous bleed of tank water rich in solids, which may be called retentate. Unfortunately, while this process preserves a mass balance, the tank water must contain a high concentration of pollutants or the process will generate large volumes of retentate.
For example, if a fully mixed continuous bleed process is operated at a recovery rate of 95% (ie. 95% of the feed water becomes filtered permeate), only 5% of the feed water leaves the tank as retentate. To preserve a mass balance of solids, the retentate must have a concentration of pollutants 20 times that of the feed water. The concentration of solids in the retentate, however, is the same as the concentration of solids in the tank since the retentate is drawn from the tank water. Accordingly, the tank water has a high concentration of pollutants at all times. Operating at a lower recovery rate, 80% for example, results in tank water having a lower concentration of solids but the cost of transporting excess feedwater and then disposing of excess retentate also increases.
Another process involves filtering in a batch mode. For example, PCT Publication No. WO 98/28066 describes a process in which retentate is not withdrawn continuously. Instead, the tank water is drained to remove the accumulated solids from time to time. The tank is then refilled with fresh feed water and operation continues. While regular operation is interrupted in this method, there is a period directly after the tank is refilled in which the membranes are operated in relatively solids lean tank water. For feed water with low suspended solids, the intervals between drainings may be long enough that the benefit gained by emptying the tank offsets the loss in production time.
With either process, as filtered water is permeated through the membranes the solids in the tank water foul the membranes. The rate of fouling is related to the concentration of solids in the tank water and can be reduced but not eliminated in a fully mixed continuous bleed process by lowering the recovery rate. Further, the solids may be present in the feed water in a variety of forms which contribute to fouling in different ways. To counter the different types of fouling, many different types of cleaning regimens may be required. Such cleaning usually includes both physical cleaning and chemical cleaning.
The most frequently used methods of physical cleaning are backwashing and aeration. These methods are typically performed frequently and thus may influence the filtering process. In backwashing, permeation through the membranes is stopped momentarily. Air or water flow through the membranes in a reverse direction to physically push solids off of the membranes. In aeration, bubbles are produced in the tank water below the membranes. As the bubbles rise, they agitate or scrub the membranes and thereby remove some solids while creating an air lift effect and circulation of the tank water to carry the solids away from the membranes. These two methods may also be combined. For example, in a fully mixed continuous bleed process as described above, aeration may be provided continuously and the membranes backwashed periodically while permeation is temporarily stopped. Alternately, PCT Publication No. WO 98/28066 mentioned above describes a process in which permeation continues for 15 minutes and then stops while the membranes are aerated for 2 minutes and 15 seconds. After the first minute of aeration, the membranes are backwashed for 15 seconds.
Chemical cleaning is typically performed less frequently than backwashing or aeration. According to one class of methods, permeation is stopped and a chemical cleaner is backwashed through the membranes. In some cases, the tank is emptied during or after the cleaning event so that the chemical cleaner can be collected and disposed of. In other cases, the tank remains filled and the amount of chemical cleaner in a cleaning event is limited to an amount that is tolerable for the application.
Known fully mixed continuous bleed processes rely heavily on aeration, backwashing and chemical cleaning to maintain membrane permeability. The cleaning methods all damage the membranes over time. In addition, backwashing with permeate or chemical cleaner interrupts permeation and reduces the yield of the process. Aeration requires energy which add to the operating costs of a reactor and the resulting circulation of tank water requires significant open space in the tank. Processes that involve frequently draining the tank require less cleaning in some cases. Particularly in large systems, however, loss in production time can be high because it is difficult to drain a large municipal or industrial tank quickly. In some cases, the tank is raised and fitted with a large number of drains to promote rapid draining but these techniques increase the cost of an installation.